Piezoelectric transducer configuration

ABSTRACT

Piezoelectric transducer elements are polarized to vibrate in a number of different planes such as longitudinally, radially, or axially. Polarized flat disk or bar transducers have been severely limited in power-handling capacity because of the inherent weakness of the usual piezoelectric materials in tension. Applicant has found that flexural disk-type elements as well as &#39;&#39;&#39;&#39;bender bar&#39;&#39;&#39;&#39; types of transducers, all of which require that the piezoelectric elements be stressed in tension, gain very substantially in power-handling capacity when a thin layer of material, such as a metal plate, is bonded to the face exposed to tensile forces. Alternately, a laminate layer of glass epoxy was also found effective to improve the power handling capabilities of the transducers, and this has been found especially useful where an electrical insulating layer is required.

United States Patent 1 3,631,383

[ 7 21 Inventor Gene Zilinskas Primary Examiner- Rodney D. Bennett, Jr.

Van Nuys, Calif. Assistant Examiner-Brian L. Ribando [21] Appl. No.844,861 Attorneys-Robert C. Smith and Plante, Arens, Hartz, Hix and [22]Filed July 25, 1969 Smith [45] Patented Dec. 28, 1971 [73] Assignee TheBendix Corporation ABSTRACT: Piezoelectric transducer elements arepolarized to vibrate in a number of different planes such as longitu-[54] PIEZOELECTRIC TRANSDUCER dinally, radially, or axially. Polarizedflat disk or bar transdu- CONFIGURATION cers have been severely limitedin ower-handlin ca acity P g P 10 C|ai ,4l) wing Fi because of theinherent weakness of the usual piezoelectric materials in tension.Applicant has found that flexural disk- [22] (3| "0431:2138 typeelements as we as bender types oftmsducers a g i 340/10 of which requirethat the piezoelectric elements be stressed in tension, gain verysubstantially in power-handling capacity [56] References Cited 7 :vhflnafthln layer 0; rtnateriall, SFCrh as axlnetal plate, isl bondetd o e acexpose o ensi e o ces. erna e y, a amma e UNITED STATES PATENTS layer ofglass epoxy was also found effective to improve the 3,094,636 6/1963Gauld 340/10 power handling Capabilities f the transducers, and this has3,202,962 8/1965 E1510" 340/10 been found especially useful where anelectrical insulating 3,249,912 5/1966 Straube.. 340 10 layerisrequirm3,255,431 6/1966 Howatt 340/10 3] 33 35 365 W 32a 3 1a 3?:

i .9 39b IILIIH m/ M 29 m ass 2% ass PIEZOELECTRIC TRANSDUCERCONFIGURATION BACKGROUND OF THE INVENTION Piezoelectric transducers havelong been used to convert electrical energy to acoustic energy and viceversa. These transducer elements deform along a given axis in responseto an electrical AC input signal, depending upon how the material ispolarized. Piezoelectric materials such as barium titanate and leadzirconates have characteristics of ceramics in that they are strong incompression, but weak in tension. Most piezoelectric transducers havebeen manufactured (poled) to vibrate longitudinally or radially.Longitudinal vibrators formed of a series of axially poled rings havebeen prestressed by being compressed between top and bottom housingmembers by means of a through bolt. Other transducers have been formedin a bar with alternate oppositely poled segments to form a bender bartype of transducer. A single segment may also act as a flexure bar or beformed as a disk which deflects in oil can fashion. Disk-type transducerelements'have had limited use because of stress limitations. Theirpowerhandling capabilities are severely limited because they normallycrack and break from physical stress long before they becomeelectrically depolarized.

SUMMARY OF THE INVENTION Disk-type piezoelectric transducers arenormally thickness polarized (along an axis perpendicular to its faces),and in order to accomplish the polarization, a very thin plating ofhighly conductive material such as silver or copper is placed on eachface, with output terminals attached to each plated surface. Numerousfailures of these disk elements at moderate power output, apparentlyfrom tensile stress during vibration, prompted applicant to bond aprestressed metal disk to the surface of the disk. This gave rise to avery substantial improvement in power output. It was later determinedthat prestressing of the metal disk was not required and that much thesame results could be obtained with an unstressed metal disk. The sametechnique was found effective with a flexural bar or with a bender barof many separate segments. In this latter case the problem of shortcircuits across the segments made it desirable to use a nonconductivereinforcing laminate, and a glass epoxy layer was also found to beeffective to permit substantially higher power output from a bar of agiven size.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows, in section, adisk-type transducer in which two disk elements are arranged back toback. One such element consists of a disk of ceramic piezoelectricmaterial 12 having a thin layer of highly conductive material such assilver or copper plated to each face. Each face thus becomes anelectrode and is connected to a source of alternating current. The faceis bonded to a backing member of metal including two parallel plates 14and 16 separated by a radially compliant spacer ring 18 leaving an airspace 20. Plates 14 and 16 are typically of about the same thickness asdisk 12. Plate i6 is bonded to a second ceramic disk 22 which isidentical to disk 12. Thin metal disk 24 and 26-ordinarily from 0.001 to0.020 inch thick, depending upon the diameter of the transducer and itsdesired frequency rangeare bonded to the faces of the ceramic disks I2and 22. Electrical leads 21, 23, 25 are connected as shown so that analternating current may be impressed across the opposite faces of theceramic disks.

The members of FIG. I are shown in an exploded view in FIG. 2. MembersI4, 16 and 18 are shown separated. These members are normally bondedtogether to form a flat cylindrical member with an airspace in thecenter. The transducer operates in the buckling or oil can" deflectionmode with each of the separate transducer elements moving outwardly andinwardly at the same time. Outward deflection places the outside surfacein tension, and without the thin metal laminate layers 24 and 26 thetransducers are limited in the power they can delivery because of thelack of tensile strength of the ceramic disks. By bonding the thin metaldisks 24 and 26 to the ceramic disks as shown, it has been found thatthe permissible deflection and hence the power-handling capacity may beincreased by up to about four times. Both prestressed and unstressedmembers 24 and 26 have been used, and both are satisfactory. It appearsthat no advantage is conferred by prestressing these disks. The bondingmay be accomplished through the use of a strong epoxy cement such asEpon VI manufactured by Shell Chemical Company. The cement should not betoo thick or stiff since this could cause breakage of the bond duringoperation and will tend to stiffen the entire transducer thus raisingits frequency and reducing its bandwidth.

The same technique is shown in FIG. 3 in connection with a bender bartype of transducer. In this type of transducer a series of bars ofpiezoelectric material 30a, 30b, 32a, 32b, 34a, 34b, 36a, 36b areconnected side by side in series and back to back such that electrodesegments 29, 31, 33, 35, 37, 39, 4!, and 43 are poled alternately. Thusupper segments are oppositely poled with respect to those immediatelybelow with the result that the upper segments tend to expand while thelower ones contract. Exactly the opposite happens in the next half-cycleso that the bar will tend to bend back and forth in synchronism withapplied alternating current. As in the case of the disk transducer, theexpansion causes tensile stress in the transducer, and ordinarily thiswould limit the power-handling capacity of the transducer on astructural basis. Because the bonding of a metal laminate directly tothe transducer faces would result in shorting the electrodes positionedbetween the segments, a layer of insulating material 44, 46 such asglass epoxy resin is bonded to each of the top and bottom faces of thesegments, and a metal laminate strip 48, 50 is then attached to eachsuch layer such that this strip is insulated from the electrodes. It hasbeen found that considerable advantage may be gained by using the glassepoxy resin alone, but for maximum strength the metal laminate should beused in combination with the insulating layer.

FIG. 4 is a sectional view of flexural bar transducer using two ceramictransducer elements 54 and 56 together. When this arrangement is used,each bar opposes the expansion of the other, thereby exciting flexuralvibrations, and thus tends to perform some of the functions of the heavymetal plates 14 and 16 shown in FIG. I. With current connected as shown,the bars 54 and 56 will tend to bow outwardly during one halfcycle andinwardly during the oppositely half-cycle. A layer of highly conductivematerial, such as silver, is plated to each side of bars 54 and 56, andeach layer is connected to an alternating-current source as shown. Thinmetal laminate bars 58 and 60 are attached to each of transducers 54 and56, respectively, and these serve to extend the amount of deflectionwhich can be tolerated by the transducer elements in the same manner asdescribed above.

It is believed that the reason for the inherent limitation inpower-handling capacity experienced with transducers having no bondedlaminate layer is that the ceramic material always has minute surfacecracks which initiate catastrophic failure as the element is caused todeflect beyond a given amount. By bonding a material having high tensilestrength-to the surface, the ceramic surface is given a greaterintegrity, the stress is distributed over a higher strength material,and growth of these cracks is inhibited to the point where structuralfailure becomes a less sever limitation on operation than doeselectrical depolarization of the transducer elements. This invention isnot limited to the specific forms of piezoelectric transducers shown butmay be used for other shapes where tensile forces tend to limit thepower-handling capacity of the transducers. Both insulating andconductive materials may be used for this purpose, as described, or bothmay be used together. It will also be apparent that the teachings of thepresent invention apply both to transmitting and receiving transducers.Thus a receiving transducer made as described may be exposed to largeracoustic forces resulting in greater deflection and greater voltageoutput through the use of elements made according to the presentinvention.

Iclaim:

1. An underwater transducer for converting electrical energy intoacoustic energy and operative in the bending mode for projecting saidacoustic energy into the surrounding water comprising at least onemember of piezoelectric material having oppositely directed faces ofsignificant area polarized such that when energized at least one of itsfaces is subject to tension forces,

a backing member of substantial stiffness positioned adjacent theopposite face of said member,

electrode means fastened to both of said opposite faces, and

a layer of material of higher tensile strength than said member bondedto said one face, said layer being sufficiently thin that it hasnegligible effect on the frequency characteristics of said transducer.

2. A transducer as set forth in claim 1 wherein said member and saidbacking member comprise disks, said backing member being of substantialthickness and being bonded to said member and said member beingpolarized and mounted such that said disks vibrate in flexure as a unitparallel to the axis of said transducer.

3. A transducer as set forth in claim 2 wherein two pairs of said disksare arranged back to back and are spaced from each other by meansproviding an airspace between said backing members, and said backingmembers are electrically connected to adjacent electrodes to form oneside of an electrical circuit and the remaining electrodes are connectedtogether to form the opposite side of said circuit.

4. A transducer as set forth in claim 1 wherein a plurality ofindividual flexural segments of piezoelectric material are alternatelypoled and mechanically positioned to form a bender bar, said segmentsare assembled in layers such that each layer acts as a backing memberfor the other, and one of said layers is bonded to each exposed face ofsaid bar, said layers being formed of high-strength insulating material.

5. A transducer as set forth in claim 4 wherein each of said layers alsoincludes a thin metal strip separated from said segments by saidinsulating material.

6. An underwater transducer for converting electrical energy to acousticenergy and operative in the bending mode for projecting said acousticenergy into the surrounding water comprising first and second thicknesspolarized disks of piezoelectric material including electrode means,

first and second disk-shaped metal backing plates bonded to one face ofsaid first and second piezoelectric disks respectively,

a radially compliant spacer member fastened to said first and secondbacking plates such that an airspace is confined between said backingplates, and

a layer of material of substantially higher tensile strength than saidpiezoelectric material bonded to each opposite face of said disks, saidlayer being sufficiently thin that it has negligible effect on thefrequency characteristics of said transducer. 7. A transducer as setforth in claim 6 wherein each said layer of material comprises a thinmetal disk.

8. A transducer as set forth in claim 6 wherein said metal backingplates are substantially the same thickness as said piezoelectric disks.

9. A method of making disk-type transducers for projecting acousticenergy into the surrounding water comprising the steps of forming afirst disk of piezoelectric material polarized such that when energizedwith an electrical alternating current signal it tends to vibrateperpendicular to its faces,

forming a backing member comprising a second disk of substantialstiffness and of essentially the same dimensions as said first disk,

bonding said first disk to said second disk,

forming a very thin disk of material having substantially greaterstrength in tension than said piezoelectric material,

and bonding said thin disk to the opposite side of said piezoelectricmaterial from said second disk. 10. A method of making piezoelectrictransducers of the type which, when energized, have at least one surfacestressed in tension comprising forming said transducer of piezoelectricmaterial to the desired shape and connecting electrodes thereto, bondinga backing member of substantial stiffness to another surface of saidtransducer, and bonding a thin layer of material having substantialstrength in tension to said one surface of said transducer.

2. A transducer as set forth in claim 1 wherein said member and saidbacking member comprise disks, said backing member being of substantialthickness and being bonded to said member and said member beingpolarized and mounted such that said disks vibrate in flexure as a unitparallel to the axis of said transducer.
 3. A transducer as set forth inclaim 2 wherein two pairs of said disks are arranged back to back andare spaced from each other by means providing an airspace between saidbacking members, and said backing members are electrically connected toadjacent electrodes to form one side of an electrical circuit and theremaining electrodes are connected together to form the opposite side ofsaid circuit.
 4. A transducer as set forth in claim 1 wherein aplurality of individual flexural segments of piezoelectric material arealternately poled and mechanically positioned to form a bender bar, saidsegments are assembled in layers such that each layer acts as a backingmember for the other, and one of said layers is bonded to each exposedface of said bar, said layers being formed of high-strength insulatingmaterial.
 5. A transducer as set forth in claim 4 wherein each of saidlayers also includes a thin metal strip separated from said segments bysaid insulating material.
 6. An underwater transducer for convertingelectrical eneRgy to acoustic energy and operative in the bending modefor projecting said acoustic energy into the surrounding watercomprising first and second thickness polarized disks of piezoelectricmaterial including electrode means, first and second disk-shaped metalbacking plates bonded to one face of said first and second piezoelectricdisks respectively, a radially compliant spacer member fastened to saidfirst and second backing plates such that an airspace is confinedbetween said backing plates, and a layer of material of substantiallyhigher tensile strength than said piezoelectric material bonded to eachopposite face of said disks, said layer being sufficiently thin that ithas negligible effect on the frequency characteristics of saidtransducer.
 7. A transducer as set forth in claim 6 wherein each saidlayer of material comprises a thin metal disk.
 8. A transducer as setforth in claim 6 wherein said metal backing plates are substantially thesame thickness as said piezoelectric disks.
 9. A method of makingdisk-type transducers for projecting acoustic energy into thesurrounding water comprising the steps of forming a first disk ofpiezoelectric material polarized such that when energized with anelectrical alternating current signal it tends to vibrate perpendicularto its faces, forming a backing member comprising a second disk ofsubstantial stiffness and of essentially the same dimensions as saidfirst disk, bonding said first disk to said second disk, forming a verythin disk of material having substantially greater strength in tensionthan said piezoelectric material, and bonding said thin disk to theopposite side of said piezoelectric material from said second disk. 10.A method of making piezoelectric transducers of the type which, whenenergized, have at least one surface stressed in tension comprisingforming said transducer of piezoelectric material to the desired shapeand connecting electrodes thereto, bonding a backing member ofsubstantial stiffness to another surface of said transducer, and bondinga thin layer of material having substantial strength in tension to saidone surface of said transducer.